Received 15 June 2007; revised 27 August 2007; accepted 7 September
2007; published 6 October 2007.

Abstract: A coupled atmosphere-ocean-carbon cycle model is used to
examine the long term climate implications of various 2050 greenhouse
gas emission reduction targets. All emission targets considered with
less than 60% global reduction by 2050 break the 2.0 threshold
warning this century, a number that some have argued represents an
upper bound on manageable climate warming. Even when emissions are
stabilized at 90% below present levels at 2050, this 2.0 threshold is
eventually broken. Our results suggest that if a 2.0Ã‚Â warming is to
be avoided, direct CO2 capture from the air, together with subsequent
sequestration, would eventually have to be introduced in addition to
sustained 90% global carbon emissions reductions by 2050.
—
========================================